The service that enables developers to add detailed annotations to the execution flow of an application, including serverless functions, is X-Ray. By using X-Ray, you can insert metadata that provides additional context within trace segments, offering insights into the application's behavior and state changes. This level of observability is crucial for effective troubleshooting. The other services mentioned do not offer this annotation capability: CloudTrail focuses on recording AWS API calls, CloudWatch primarily deals with monitoring and metrics, and Step Functions orchestrate different serverless components rather than provide annotations for tracing.